This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. This is a double-blind, placebo controlled study testing the hypothesis that chronic PDE-5 inhibition (sildenafil 80 mg tid for 24 weeks) improves exercise capacity and clinical status in patients with heart failure and normal ejection fraction. This study also measures the effect of this therapy on key pathophysiological parameters which are postulated to impact clinical status and exercise performance in diastolic heart failure. 120 patients will be studied. Approximately 10 here at FAHC. The primary endpoint is change in exercise capacity as assessed by peak VO2 after 24 weeks of double-blinded treatment PDE-5 inhibitor or placebo.